The present invention relates to an image forming apparatus, such as a laser printer, laser duplicator, laser facsimile, and the like, which forms images having a halftone by means of multiple-value image data, and to a method therefor. The multi-value image data is such data that 256 levels from "00H" to "FFH" (`H` means hexadecimal number, that will be the same as hereinafter) are allocated to halftone levels from white to black.
Recently, as a high speed low noise printer, a laser beam printer using an electronic photo method has drawn the attention of the public. The laser beam printer is utilized mainly for forming a two value image, having only two tones (white and black), such as letters, line drawings, figures, and the like. In this printer, normally a halftone image is not treated. Therefore, the construction of the printer or image processing circuits of the printer can be simplified. Also, by utilizing this two value printer, a halftone image can be formed by means of the known Dizzer method or Dot pattern method.
However, by utilizing the two value printer using the dither method or Dot pattern method, a halftone image having a high resolution can not be formed. Recently, a printer forming a halftone image by utilizing the two value printing method has been developed. In this printer, an image signal is modulated in a pulse width modulation(PWM) manner, and then a laser is driven by the modulated image signal. This printer using the PWM method can be effectively used for forming color images.
Japanese Laid-Open Patent Application No. 2-192966 discloses one example of a PWM(pulse width modulation) circuit in such a conventional image forming apparatus. The composition of the circuit and main signals thereof are shown in FIG. 1 and FIGS. 2A to 2F respectively. In FIG. 1 and FIGS. 2A to 2F, a 8-bit parallel image signal `a` having a transistor transistor logic(TTL) level is latched by TTL latch circuit 101. Subsequently, this signal `a` is then converted into a signal having an emitter coupled logic(ECL) level by a level converter 102. Subsequently, the image signal having an ECL level is then converted into an analogue signal `b` by ECL D/A(Digital/Analogue) converter 103. Further, this signal `b` is supplied to one input terminal of a ECL comparator 104.
A clock oscillator(OSC) 106 generates clock signal(2f) having a frequency of 2f. A triangular wave signal generator 107 generates a generally ideal triangular wave signal(pattern signal having the same pattern as each other per each period) serving as a reference signal in synchronization with the clock signal(2f). This triangular wave signal is supplied to the other input terminal of the ECL comparator 104. 1/2 frequency divider 108 divides this clock signal 2f into half. Further, the 1/2 frequency divider 108 supplies an image clock signal (f) having a frequency f and a duty ratio of 50% to the TTL latch circuit 101. The TTL latch circuit 101 latches the parallel image signal `a` in synchronization with this image clock signal(f). Therefore, both of a period of the image clock signal(f) shown in FIG. 2C and a period of the triangular wave signal shown in FIG. 2D are synchronized with a period of each pixel shown in FIG. 2A.
ECL comparator 104 supplies a PWM signal having an ECL level corresponding to a signal level differential between the analogue signal `b` converted by the D/A converter 103 and the triangular wave signal supplied by the triangular wave signal generator 107. A level converter 105 converts the ECL level of the PWM signal into the TTL level. A laser driver circuit 109 turns a laser diode 110 on and off, which diode emits light to a photosensitive drum corresponding to a pulse width of the PWM signal. Thus, an electrostatic latent image is formed on the photosensitive drum.
However, in the above mentioned conventional image forming apparatus, an image forming speed depends on the frequency `f`. Therefore, the frequency `f` has to be increased to increase the image forming speed. For example, if the frequency `f` is set at 5 MHz, the shared time for one pixel becomes not more than 200 nsec. Forming an ideal triangular wave, such as that shown in FIG. 3A, in such a short period is difficult. In addition, forming the triangular wave in such a short period results in distortion of the triangular wave increasing, as shown in FIG. 3B.
A characteristic of a pulse width of the PWM signal corresponding to an input image signal `a` is defined in relation to a waveform of utilized reference signals, as will be described below. In a case where an ideal reference signal, such as that shown in FIG. 3A, is utilized, a pulse width varies linearly, as is indicated by a broken line in FIG. 4. In a case where a distorted reference signal, such as that shown in FIG. 3B, is utilized, a pulse width does not vary linearly, as is indicated by solid line in FIG. 4. Further, in an image forming apparatus utilizing an electrical photo processing method, an image tone level corresponding to a pulse width of the PWM signal does not vary linearly, as is indicated by a line in FIG. 5, due to characteristics of an image tone level forming means. To correct such a characteristic shown in FIG. 5, which is not linear, a characteristic curve for converting an input image signal into the PWM signal should be set as shown in FIG. 6 beforehand.
Further, in Japanese Laid-Open Patent Application No. 62-49781, a method for varying a period of a pulse signal corresponding to a letter image and photo image without varying the amplitude nor bias thereof has been proposed. However, even if this method is utilized, due to an increase in the image forming speed, a reference signal having a desired shaped wave cannot be produced.